Conventionally, lubricating oils used for automatic transmissions, manual transmissions or internal combustion engines have been required to be enhanced in various durability-related properties such as thermal oxidation stability, anti-wear properties, anti-fatigue properties or the like and to be improved in viscosity temperature characteristics so as to improve the fuel saving properties and in low temperature viscosity characteristics such as reduction in low temperature viscosity and improvement in low temperature fluidity. In order to improve these properties, a lubricating oil has been used, whose base oil is appropriately blended with various additives such as an antioxidant, a detergent dispersant, an antiwear agent, a friction modifier, a seal swelling agents, a viscosity index improver, an anti-foaming agents, a colorant and the like.
Recent transmissions and engines have been demanded to be fuel efficient, small and light and be increased in power output. Transmissions have been sought to be improved in power transmission capability in connection with the increased power output of the engines with which the transmissions are used in combination. Lubricating oils to be used for such transmissions have been, therefore, demanded to maintain high lubricity and to possess properties to prevent the wear or fatigue of surfaces of bearings and gears while reduced in the product viscosity and base oil viscosity as well as anti-seizure properties. Generally, in order to improve the fuel saving properties of a lubricating oil, a technique has been employed, wherein the viscosity temperature characteristics are improved by reducing the base oil viscosity and increasing the amount of a viscosity index improver. However, the reduction in the base oil viscosity degrades the anti-fatigue properties. Therefore, the development of a lubricating oil has been eagerly desired, which can achieve not only good fuel saving properties but also anti-wear/anti-seizure properties and anti-fatigue properties.
Under these circumstances, it has been known to use a base oil with good low temperature properties or a base oil with a high viscosity in combination, or to blend a phosphorus- or sulfur-based extreme pressure additive in a suitable amount in order to improve not only the fuel saving properties but also the low temperature viscosity characteristics and anti-fatigue properties (see, for example, patent documents 1 to 3 below).
However, the techniques disclosed in these patent documents are not sufficient to improve the viscosity temperature characteristics, and low temperature properties as well as anti-fatigue properties and anti-seizure properties. A lubricating oil composition has been, therefore, demanded to be developed, which has all of these properties and characteristics but having no problem in other properties.